Method of clearing a fault in a HVDC electrical network

1. A method of clearing a fault in a high voltage DC electrical network including at least three power converters interconnected by at least a first DC power transmission medium, the method comprising: detecting a fault in the first DC power transmission medium and in response reconfiguring each power converter from a normal power transmission mode to a fault blocking mode to drive the resulting DC fault current flowing in the first DC power transmission medium towards zero; locating the fault in the first DC power transmission medium and isolating a faulty portion of the first DC power transmission medium from a healthy remaining portion of the first DC power transmission medium; reconfiguring at least one of the power converters designated as a re-energising power converter from the fault blocking mode to a re-energising mode to re-energise the healthy remaining portion of the first DC power transmission medium; and detecting a rise in the voltage level with respect to earth in the healthy remaining portion of the first DC power transmission medium above a first threshold level and in response reconfiguring the remaining power converter connected with the healthy remaining portion of the first DC power transmission medium from the fault blocking mode to the normal power transmission mode.

2. The method of clearing a fault according to claim 1 , wherein the at least three power converters are interconnected by first and second DC power transmission mediums, and wherein the step of detecting a fault in the first DC power transmission medium includes detecting a difference between the voltage levels in each of the first and second DC power transmission mediums.

3. The method of clearing a fault according to claim 1 , wherein the at least three power converters are interconnected by first and second DC power transmission mediums, and wherein the step of detecting a fault in the first DC power transmission medium includes detecting a rise in current flowing in each of the first and second DC power transmission mediums.

4. The method of clearing a fault according to claim 1 , wherein the at least three power converters are interconnected by only a first DC power transmission medium, and wherein the step of detecting a fault in the first DC power transmission medium includes detecting a rise in current flowing in the first DC power transmission medium.

5. The method of clearing a fault according to claim 2 , wherein each of the first and second DC power transmission mediums has at least one energy removal device connected therewith, and wherein after the step of reconfiguring each power converter from the normal power transmission mode to the fault blocking mode to drive the resulting DC fault current flowing in the first DC power transmission medium towards zero, the method includes the step of activating at least one of the energy removal devices connected with the second DC power transmission medium to remove energy from the second DC power transmission medium and thereby reduce the voltage level in the second DC power transmission medium from an overvoltage level following occurrence of the fault to a second threshold level below a normal absolute operating voltage level of the second DC power transmission medium.

6. The method of clearing a fault according to claim 5 , wherein the second threshold level is additionally below the first threshold level.

7. The method of clearing a fault according to claim 5 wherein after the step of activating at least one of the energy removal devices connected with the second DC power transmission medium, the method includes the step of deactivating the said energy removal device for a predetermined period of time, the deactivation of the said energy removal device coinciding at least in part with the step of locating the fault in the first DC power transmission medium and isolating the faulty portion of the first DC power transmission medium from the healthy remaining portion of the first DC power transmission medium.

8. The method of clearing a fault according to claim 7 , wherein the step of deactivating the said energy removal device connected with the second DC power transmission medium for a predetermined period of time, is followed by the step of selectively activating at least one of the energy removal devices connected with the first DC power transmission medium and selectively activating at least one of the energy removal devices connected with the second DC power transmission medium to selectively interconnect the first and second DC power transmission mediums and thereby provide a path via which the voltage levels in the first and second DC power transmission mediums are able to rebalance.

9. The method of clearing a fault according to claim 8 , wherein the step of reconfiguring at least one of the power converters designated as a re-energising power converter from the fault blocking mode to the re-energising mode takes place on detection of balanced voltage levels in the first and second DC power transmission mediums.

10. The method of clearing a fault according to claim 3 , wherein the step of reconfiguring at least one of the power converters designated as a re-energising power converter from the fault blocking mode to a re-energising mode takes place a predetermined period of time after the fault is detected.

11. The method of clearing a fault according to claim 1 wherein the step of locating a fault in the first DC power transmission medium includes interrogating a plurality of DC protection elements located throughout the electrical network, the DC protection elements being so located throughout the electrical network so as to permit the selective isolation of respective portions of a given DC power transmission medium from one another.

12. The method of clearing a fault according to claim 1 , wherein the step of isolating a faulty portion of the first DC power transmission medium from the healthy remaining portion of the first DC power transmission medium includes the step of operating at least one DC protection element lying adjacent to the faulty portion to electrically separate the said faulty portion from the healthy remaining portion.

13. The method of clearing a fault according to claim 12 , wherein the step of isolating a faulty portion of the first DC power transmission medium from the healthy remaining portion of the first DC power transmission medium additionally includes the step of isolating a faulty portion of the second DC power transmission medium from a healthy remaining portion of the second DC power transmission medium by operating at least one DC protection element lying adjacent to the said faulty portion of the second DC power transmission medium to electrically separate the said faulty portion from the healthy remaining portion of the second DC power transmission medium.

14. The method of clearing a fault according to claim 12 wherein operating the said DC protection device takes place once the DC fault current flowing through the corresponding DC power transmission medium has been driven down by the fault-blocking power converters below a safety threshold level.